The present invention relates to means for dispersion compensation required for optical communication systems, and specifically to a new device for adjusting the distribution of Bragg gratings along an optical fiber.
When modulated light is transmitted through a single mode optical fiber, one of the difficulties is the dispersion of light within the optical fiber. To cope with this difficulty, a dispersion compensation fiber (DCF) has been proposed. However, the DCF has a drawback in that the light transmitting in the DCF suffers a relatively large attenuation. To provide an alternative, an optical fiber with in-fiber Bragg gratings has been developed, in which the interval of the gratings varies continuously depending on the location of the gratings along the optical fiber (hereinafter a xe2x80x9cchirped fiberxe2x80x9d).
The inventor of the present invention owns U.S. Pat. No. 6,160,261 in which an efficient method of producing the chirped fiber is described. Patent applications on the same invention have been made in several countries including Japan. The invention of the above-mentioned U.S. Patent enables production of a chirped fiber having a length over 10 meters with a desired distribution of the gratings. With the patented method, it is possible today to design and produce a dispersion compensator adapted for a communication line of long distance, working under a given spectrum of light, with given dispersion characteristics of the optical fiber and so forth taken into consideration. A dispersion compensator provided with a long chirped fiber over 10 meters is suited for, for instance, a long distance intercity optical communication line.
In recently built optical communication networks, the optical fibers included as network components are switched by wave-guide switches, for instance, to select and combine them with each other. In such a situation, it is desired to compensate for, as quickly and adaptively as possible, the dispersion in respective optical fibers used in a narrow band such as a single channel in the WDM system.
To practice the adaptive dispersion compensation mentioned above, it is necessary to be able to freely adjust the interval of the gratings included in the dispersion compensators. There have been many proposals until now about how to realize an adjustable chirped fiber.
A method for producing an adjustable chirped fiber by utilizing thermal expansion/contraction is presented in John A. Rogers et al; Distributed on-fiber thin film heaters for Bragg gratings with adjustable chirp, American Inst. of phys., Vol 74, No. 21, May 24, 1999. In this method, the optical fiber is plated with metal, and the thickness of the metal layer is controlled so that a temperature distribution is produced in the axial direction of the optical fiber with the electric current applied to the metal layer, thereby changing the distribution of the gratings in the optical fiber.
As well as those utilizing physical phenomena other than thermal expansion, there is at least one with piezoelectricity (Electro. Lett. Vol. 34, No. 24, Nov. 24th, 1998), and another which relies on magnetostriction (Electro. Lett. Vol. 33, No. 3, Jan. 30th, 1997).
There are several examples in which bending of a beam is utilized. M. L. Blanc et al; Tunable chirping of a fibre Bragg grating using a tapered cantilever bed, Electro. Lett. Vol. 30, No. 25, Dec. 8th, 1994, which discloses a cantilever whose width and thickness is gradually reduced in a taper toward the root thereof. A groove is formed on the top surface of the cantilever, and the optical fiber is fixed with adhesive to the bottom of the groove. When the cantilever is loaded with a bending force at the tip thereof, the optical fiber is subjected to a stress, the magnitude of which varies linearly in the axial direction of the optical fiber. With the bending force of a certain magnitude, the distribution of the gratings in the optical fiber can be maintained in a corresponding state. However, the wave-length of light reflected by the mid-point of the optical fiber, namely the central wave-length, shifts depending on the magnitude of the bending force. Therefore, a disclosed fiber Bragg grating is inconvenient for use in dispersion compensation.
T. Imai et al; Dispersion Tuning of a Linearly Chirped Fiber Bragg Gratings Without a Center Wavelength Shifts by Applying a Strain Gradient, IEEE photonics Technology Lett. Vol. 10, No. 6, June 1998, discloses another example in which bending of a beam is utilized. In this case, an optical fiber is held in a plastic cylinder which is adhered to the top surface of a metal beam.
The beam is given a set of bending moments rotating in the same direction at each end of the beam so that the beam is deformed in an S figure. The deformation is null at the mid-point of the beam, whereas, it becomes gradually larger toward both ends of the beam, with a stretching stress exerting on the top surface on one side of the mid-point, and a compressing stress on the other side.
Therefore, while the distribution of the gratings in the optical fiber changes depending on the magnitude of the bending moments, the shifts of the central wave-length do not occur.
In the disclosed beam, a set of bending moments rotating in the same direction are produced by shifting both clamps gripping the ends of the beam, raising one clamp and lowering the other in the vertical direction. Some slippage of the optical fiber in the cylinder, presumably due to incomplete adhesion, was reported.
A common requirement observed in the conventional technologies raised above is to provide the strain to be applied to the optical fiber with a gradient. In the conventional technology 1, an elaborate means was devised to produce the gradient in the thermal strain (or piezoelectric, magnetostrictive strain), essentially, to have no connection with the gradient. In the conventional technologies 2 and 3 special means such as a unique shape of the beam, or a unique way of moving the clamps, were devised. With these, complexity was introduced into the respective structures.
Specifically speaking with the conventional technology 3, in the disclosed way of shifting the clamps to produce the bending moments, a drawback may arise in that a substantial distance change between the clamps could be created by shifting, and an axial tension caused in the beam. This would cause a shift in the point of no expansion or compression in the optical fiber. Further, if the clamps made contact with the beam in the small contact areas, there would be a possibility that the contact state between the clamps and the beam would become unstable due to fatigue of the material. This in turn would affect the balance of the bending moments at both ends of the beam.
How to fix the optical fiber in the beam is another problem in the conventional technologies 2 and 3. In the respective ways of fixing the optical fiber in the beam disclosed, the radial symmetry of the optical fiber in the substance of the beam would not necessarily be secured, and this would make it difficult to tell what kind of force was actually working on the optical fiber.
Still another problem is how to compensate for the thermal expansion of the beam. Plastics as the material of the beam generally show a far greater expansion coefficient than that of metals. In contrast, a metal beam requires a far greater force ell to produce a predetermined strain on the optical fiber, compared to a plastic beam. It is therefore necessary to find how to compensate for the thermal expansion of the plastic beam. However, there is no such disclosure in the conventional technologies.
The present invention provides a simple device for dispersion compensation, in which an optical fiber provided with normally (when no bending force is loaded on the beam) uniform gratings fixed in a beam is subjected to tension or compression when the beam is bent. This depends on the axial location in the beam of the observed point of the optical fiber, and as a result, the interval of the gratings can be varied continuously in the axial direction of the optical fiber (in short, xe2x80x9cchirpedxe2x80x9d). Chirping of the gratings may be adjusted by adjusting the extent of the bending of the beam.
The device according to the present invention comprises:
a straight beam having a longitudinally uniform cross-section, uniformly bent by external force in a vertical plane including the axis of the beam, and
an optical fiber provided with gratings, extending straight along said vertical plane in the beam when the beam is not loaded with the external force, from the top surface at one end to the bottom surface at the other end of the beam, penetrating on the way the neutral surface of the beam.
When a straight beam is bent, a strain is produced in the beam, the magnitude of which varies linearly, from negative to positive values depending on which side of and how distant it is from a neutral surface. If an optical fiber is fixed obliquely in the beam so that it penetrates the neutral surface on its way, portions of the optical fiber existing on the convex side of the neutral surface in the bent beam are subjected to tension, and on the concave side, compression, whereas no tension nor compression is present at the cross-point with the neutral surface.
Accordingly, if an optical fiber placed in the beam is provided with gratings of uniform interval, the uniform gratings change into chirped gratings, by simply bending the beam, while the intervals vary continuously in the axial direction. As a result, the range of wave-lengths of light reflected by the gratings is expanded from a sharp point to a band including higher and lower wave-lengths on both sides of a central wave-length. However, in the course of changing, the central wave-length remains unchanged, because, the light of the central wave-length is reflected by the optical fiber at or near the cross-point with the neutral surface, where the interval of the optical fiber remains unchanged.
If the optical fiber is provided with gratings chirped in advance, it reflects light of a predetermined band of wave-lengths, without bending of the beam. However, it is possible to adjust the bandwidth by exerting an appropriate bending force upon the beam.
In the above-mentioned device, as the optical fiber is buried deep in the substance of the beam, the radial symmetry of the optical fiber is secured, and there is little boundary effect between the beam and the optical fiber. The optical fiber is given almost genuine axial force from the environment, and is dynamically stable. Further, as the radial symmetry of the optical fiber is stably maintained, there is little danger of suffering from a polarization mode dispersion (PMD), which is caused by polarized light speeds dependent on the asymmetric strain of the fiber glass.
As stated above, in the device according to the present invention, the distribution of gratings in the axial direction of the optical fiber can be easily adjusted by a simple procedure such as exerting a uniform bending force onto the straight beam. This device is suited, for instance, for quickly and adaptively compensating for dispersion suffered by an individual narrow band channel such as a single channel in a WDM system.
Fifteen preferred embodiments are provided in the description. The first to eighth embodiments show variations to the above-mentioned device. The ninth and tenth embodiments show temperature compensating means applicable to the above-mentioned device. The eleventh and twelfth embodiments show similar devices including beams which have special cross-sections. The thirteenth to fifteenth embodiments show methods for producing the disclosed devices.